docs: def Hwang_Kim(m, x, rhol, rhog, mul, mug, sigma, D, roughness=0, L=1):
      r'''Calculates two-phase pressure drop with the Hwang and Kim (2006)
      correlation as in [1]_, also presented in [2]_ and [3]_.
  
      .. math::
          \Delta P = \Delta P_{l} \phi_{l}^2
  
      .. math::
          C = 0.227 Re_{lo}^{0.452} X^{-0.32} Co^{-0.82}
  
      .. math::
          \phi_l^2 = 1 + \frac{C}{X} + \frac{1}{X^2}
  
      .. math::
          X^2 = \frac{\Delta P_l}{\Delta P_g}
  
      Parameters
      ----------
      m : float
          Mass flow rate of fluid, [kg/s]
      x : float
          Quality of fluid, [-]
      rhol : float
          Liquid density, [kg/m^3]
      rhog : float
          Gas density, [kg/m^3]
      mul : float
          Viscosity of liquid, [Pa*s]
      mug : float
          Viscosity of gas, [Pa*s]
      sigma : float
          Surface tension, [N/m]
      D : float
          Diameter of pipe, [m]
      roughness : float, optional
          Roughness of pipe for use in calculating friction factor, [m]
      L : float, optional
          Length of pipe, [m]
  
      Returns
      -------
      dP : float
          Pressure drop of the two-phase flow, [Pa]
  
      Notes
      -----
      Developed with data for microtubes of diameter 0.244 mm and 0.792 mm only.
      Not likely to be suitable to larger diameters.
  
      Examples
      --------
      >>> Hwang_Kim(m=0.0005, x=0.1, rhol=915., rhog=2.67, mul=180E-6, mug=14E-6,
      ... sigma=0.0487, D=0.003, roughness=0, L=1)
      798.302774184557
  
      References
      ----------
      .. [1] Hwang, Yun Wook, and Min Soo Kim. "The Pressure Drop in Microtubes
         and the Correlation Development."  International Journal of Heat and
         Mass Transfer 49, no. 11-12 (June 2006): 1804-12.
         doi:10.1016/j.ijheatmasstransfer.2005.10.040.
      .. [2] Kim, Sung-Min, and Issam Mudawar. "Universal Approach to Predicting
         Two-Phase Frictional Pressure Drop for Adiabatic and Condensing Mini/
         Micro-Channel Flows." International Journal of Heat and Mass Transfer
         55, no. 11-12 (May 2012): 3246-61.
         doi:10.1016/j.ijheatmasstransfer.2012.02.047.
      .. [3] Xu, Yu, Xiande Fang, Xianghui Su, Zhanru Zhou, and Weiwei Chen.
         "Evaluation of Frictional Pressure Drop Correlations for Two-Phase Flow
         in Pipes." Nuclear Engineering and Design, SI : CFD4NRS-3, 253 (December
         2012): 86-97. doi:10.1016/j.nucengdes.2012.08.007.
      '''
      # Liquid-only flow
      v_lo = m/rhol/(pi/4*D**2)
      Re_lo = Reynolds(V=v_lo, rho=rhol, mu=mul, D=D)
  
      # Actual Liquid flow
      v_l = m*(1-x)/rhol/(pi/4*D**2)
      Re_l = Reynolds(V=v_l, rho=rhol, mu=mul, D=D)
      fd_l = friction_factor(Re=Re_l, eD=roughness/D)
      dP_l = fd_l*L/D*(0.5*rhol*v_l**2)
  
      # Actual gas flow
      v_g = m*x/rhog/(pi/4*D**2)
      Re_g = Reynolds(V=v_g, rho=rhog, mu=mug, D=D)
      fd_g = friction_factor(Re=Re_g, eD=roughness/D)
      dP_g = fd_g*L/D*(0.5*rhog*v_g**2)
  
      # Actual model
      X = (dP_l/dP_g)**0.5
      Co = Confinement(D=D, rhol=rhol, rhog=rhog, sigma=sigma)
      C = 0.227*Re_lo**0.452*X**-0.320*Co**-0.820
      phi_l2 = 1 + C/X + 1./X**2
      return dP_l*phi_l2